A known device for testing a brake fluid and detecting its boiling point comprises a heating cup into which some of the fluid is injected with the aid of a syringe. After being closed against the atmosphere, the cup is heated until evolving gas bubbles begin to pass through a U-shaped tube into a receptacle therefor. The temperature at which this occurs is noted as the boiling point, whereupon the liquid in the cup is returned to the brake system of the vehicle by means of the same syringe; both the cup and the syringe are then carefully cleansed preparatorily to a new test. Such a device is relatively expensive and cumbersome to handle, being thus not very well suited for use in a repair shop for automotive vehicles.